Transvenous defibrillator leads are useful for the correction of ventricular tachycardia and ventricular fibrillation. Leads of this type are intravenously positioned so that the electrode portion of a lead is located within the right side of the heart. The lead may have only a single conductive electrode surface at or near the distal tip of the lead which is intended to be used in conjunction with an additional, separate and independent electrode such as a patch electrode located subcutaneously on the left side of the body. Alternatively, the transvenous defibrillator lead may incorporate two separate electrodes at or near the distal tip of the lead which may be used in conjunction to deliver electrical energy to the heart. More than two electrodes may be provided within the distal tip portion if it is desired to provide electrodes for sensing as well as for delivering electrical energy.
Conventional transvenous defibrillator leads use a helically wound wire to conduct the electrical energy from the connector at the proximal end of the lead to the electrode at the distal end. Multiple conductor wires typically are in the form of separate helically wound wires in coaxial relationship wherein each wire is separated from an adjacent wire by a tubular insulating layer. Alternatively they may be in the form of a co-linear helical winding wherein the individual wires are individually insulated prior to winding into a single helical form.
The conductive electrode surface is most commonly provided by leaving a length of the helically wound wire uninsulated and exposed to allow it to be exposed to the interior surface of the heart. While using the helically wound wire has the advantage of eliminating a connection between a separate electrode and the conductor wire, it has a fundamental disadvantage in that tissue grows into the exposed helically wound wire over time with the result that the lead can be extremely difficult to remove by the application of tension to the proximal end of the lead.
Various methods have been attempted to overcome this difficulty. For example, U.S. Pat. No. 5,090,422 describes the use of a porous covering for use over the electrode surface wherein the covering is made of a biocompatible material which may be an insulating material but becomes conductive by virtue of penetration of the material by conductive body fluids. The porous covering is of adequately small pore size to preclude substantial tissue ingrowth. Recommended materials include woven, porous polyurethane and porous polytetrafluoroethylene if used with a wetting agent or surface modifier.